The present invention relates to a method of stopping a winding machine which is used for winching a crane rope up and down.
Winding machines, which use electric motors for driving and electromagnetic brakes for stopping, manage the critical timing of the operation of the motor and electromagnetic brake during the transition from working to stopping. Far example, when an operator wishes to winch down a load with a crane rope and stop the crane when the load reaches a certain height, if the activation of the electromagnetic brake comes after the motor stops, the crane rope will temporarily be in a no-torque state, resulting in the dangerous descent of the load.
To deal with this matter, Unexamined Japanese Patent publication No. Sho 59-124690 discloses a method of controlling the timing of the operation of the motor and electromagnetic brake in which the electromagnetic brake is activated immediately before the motor stops, thereby halting the descent of load. FIG. 1 is a block diagram of the control circuit of this prior art, and FIG. 2 is the timing chart at stopping.
In the figure, a winch induction motor M1 has its output shaft coupled at one end to a drum of an electromagnetic brake MB and at another end to a winch drum D through a reduction gear G. Connected between a primary winding of the induction motor M1 and the power source is a variable-voltage, variable-frequency inverter I1. which is also connected to the electromagnetic brake MB by way of a brake controller BC.
The operation of the system will be explained with reference to FIG. 2. When a stop command SR is issued to the inverter h at time t7, the speed command NREF to the induction motor MI decreases, and the motor speed NFB falls accordingly. When the motor speed NFB has fallen to a prescribed level (immediately before the stop), a brake command BR is issued to the electromagnetic brake MB at time t8 to activate it.
In the above prior art method of stopping a winding machine, the electromagnetic brake is activated during the rotation of the motor, and therefore has the problem of prematurely wearing the electromagnetic brake. Another problem is that the control ceases at time is when the speed command NREF reaches zero, causing the current command to become zero, even if the electromagnetic brake is not activated, in which case the descent of the load will occur.
Accordingly, an object of the present invention is to prevent abrasion of the electromagnetic brake and the descent of the load.
In order to achieve the above objective, the inventive method of stopping a winding machine which operates to winch a load up or down in response to a torque command applied to an electric motor and produced by a speed controller in accordance with a speed command generated by a speed command generation circuit, the method comprising the steps of: decreasing the speed command to the motor to zero for a predetermined length of time at the stopping of the motor, thereby the torque command to the motor is controlled to zero; braking the motor by issuing a brake command to an electromagnetic brake which is coupled directly to the motor after the motor speed NFB detected by the speed detector is zero and turning off a power supply to the motor if the motor speed NFB detected by the speed detector is zero.
In the inventive method of stopping a winding machine, if the motor does not rotate when the torque command is reduced to zero for a predetermined length of time following the activation of the electromagnetic brake after the motor has stopped completely based on the operation of the above-mentioned means, it can be judged that the electromagnetic brake is producing a torque sufficient to withstand the load at that time, and therefore the descent of load can be prevented even in the absence of the motor control. By activating the electromagnetic brake during the stoppage of the motor, the brake will incur abrasion and the descent of load is prevented.